In the manufacture of aluminum alloy structures, there are a number of important processes, such as adhesive bonding and anodizing, which require that the aluminum surfaces be free from contaminants and undesirable oxidation at the beginning of the process. Since the initial handling of alloy stock almost invariably results in contamination and/or oxidation of the surfaces, aluminum alloy articles that are to be subjected to these processes must be cleaned prior to the processes. One type of process that requires a clean aluminum alloy article is phosphoric acid anodizing to form a controlled aluminum oxide coating suitable as a base for an adhesive bond. An example of this type of process is disclosed in U.S. Pat. No. 4,085,012, granted Apr. 18, 1978, to J. A. Marceau et al. The Marceau et al. process and similar processes require preliminary cleaning nd deoxidation to provide a clean surface with a controlled oxide composition for the anodizing process to ensure the proper formation of the aluminum oxide coating and, thereby, ensure the quality of the adhesive bond.
The cleaning and deoxidizing procedures that are currently in use present a number of serious problems. A widely used type of solution is one which contains sulfuric acid and large amounts of chromic acid. This type of solution is effective in cleaning the aluminum alloy surfaces, but the presence of hexavalent chromium (Cr.sup.+6) in the solution complicates the cleaning procedure and greatly increases its cost. Since hexavalent chromium can present a health hazard, extensive safety precautions must be used during the use of the solution. In addition, waste disposal and treatment of large quantities of dilute wash water effluent are complicated and very costly because of the need to strictly limit introduction of hexavalent chromium into the environment.
Hot solutions of strong acids that are chromium free are a possible alternative to currently used chromic acid solutions. These solutions might contain concentrated sulfuric and/or nitric acid and other additives, such as detergents and oxidizers like ferric sulfate. This type of solution would not present safety and environmental problems of the same severity as a chromic acid solution, but the temperature of the solution and the strength of the acids would still involve significant safety and environmental problems. High solution temperatures also increase the cost of the procedure by increasing heating costs. In addition, it would be difficult to obtain by use of such solutions the same slow predictable etch rate that is obtainable using chromic acid solutions. Moreover, hot solutions of strong acids can cause intergranular attack (pitting) on the surfaces of the articles being cleaned and smut formation on such surfaces caused by redeposition of dissolved copper.
U.S. Pat. No. 4,097,342, granted June 27, 1978, to W. E. Cooke et al. discloses an electrolytic cleaning treatment for aluminum stock prior to metal plating. The treatment is carried out under anodic conditions in a high temperature solution of strong acid for the minor part of a minute. For a solution of 37% phosphoric acid and 18% sulfuric acid, a temperature range of 176.degree. to 203.degree. F. is described as satisfactory. Other solutions and temperatures may also be used provided that a dissolving power is maintained similar to that of the phosphoric acid and sulfuric acid solution. Cooke et al. state that ideally the anodic oxide is removed from the aluminum as rapidly as it forms. The cleaning process may also include a nonelectrolytic treatment for one or two seconds in the same or a similar bath before and/or after the electrolytic treatment.
U.S. Pat. No. 2,708,655, granted May 17, 1955, to H. L. Turner discloses a process for removing an oxide film left by a polishing step before anodizing an aluminum article. The process includes immersing the article in a solution of chromic and phosphoric acids or chromic and sulfuric acids. U.S. Pat. No. 2,721,835, granted Oct. 25, 1955, to W. G. Axtell discloses a process for treating an aluminum article prior to painting or enameling. The process includes subjecting the article to electrolytic treatment in a solution of phosphoric and chromic acid. Axtell describes the effect on an oxide layer on the aluminum article as apparently being removing a portion of it and leaving a comparatively spongy layer which is permeated by the solution and dissolves in the subsequently applied coat of paint or enamel. U.S. Pat. No. 3,041,259, granted June 26, 1962, to W. B. Stoddard, Jr. discloses an alkaline electrolytic process for cleaning aluminum without degrading the surface finish.
Processes for forming controlled oxide coatings on aluminum surfaces are disclosed in the Marceau et al. patent cited above, French Patent Application No. 2,360,051, made public on Feb. 24, 1978, and U.S. Pat. No. 3,844,908, granted Oct. 29, 1974, to H. Matsuo et al; U.S. Pat. No. 3,915,811, granted Oct. 28, 1975, to R. A. Tremmel et al.; U.S. Pat. No. 4,022,671, granted May 10, 1977, to T. Asada; U.S. Pat. No. 4,440,606, granted Apr. 3, 1984, to J. H. Powers et al.; U.S. Pat. No. 4,448,647, granted May 15, 1984, to T. N. Gillich et al.; and U.S. Pat. No. 4,452,674, granted June 5, 1984, to T. N. Gillich et al.
The above-cited patents and the prior art that is discussed and/or cited therein should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.